Ubuntu Buttons for Gnome Panel
HOW TO INSTALL
On a gnome powered Desktop, just run gconf-editor from a terminal, then navigate in apps->panel->objects and locate the object about your menu. Once you located it , simply 'tick' the parameter called 'use custom icon' and then put the path to the image in the field 'custom icon'.
Two versions are available for download, one with black text (for light background) and one with white text (for dark background)